1. Field of the Invention
The invention relates to a method of fabricating a semiconductor device, and more particularly to a method of fabricating a contact plug.
2. Description of the Related Art
As the integration of integrated circuits increases, the wafer cannot provides enough surface area for interconnects. To meet the requirement of interconnects for a metal-oxide-semiconductor (MOS) with a reduced dimension, multilevel metallization has become a popular way adapted in many integrated circuit devices. Normally, between a device and a metal layer, a dielectric layer is formed for isolation. The MOS electrodes and the metal layers are then electrically connected by a contact plug. However, to obtain good junction properties, such as adhesion, between the contact plug, the electrode and metal layers, a barrier layer is formed by good conductive material before the formation of the plug.
Titanium nitride is the most broadly used barrier material in a highly integrated circuit. To improve the Ohmic contact between metal and silicon, titanium nitride barrier is formed accompanied with a titanium layer as a form of titanium/titanium nitride (Ti/TiN) layer. For example, in the fabrication process of an alloy contact, the metal structure is assembled by titanium, titanium nitride, and alloy. Therefore, the work function of the contact interface is decreased, and the spike and electromigration are suppressed.
In a conventional method of fabricating a barrier layer and an interconnect, a titanium layer is formed first. Using physical vapor deposition (PVD), a titanium nitride layer having a thickness of about 800 .ANG. to 1200 .ANG. is formed as a barrier layer on the titanium layer. A refractory and well conductive metal, for example, a tungsten layer, is formed on the titanium barrier layer. After etch back, a plug formed of the metal layer for interconnecting metal layers is formed. The surface of the device is then cleaned for the another metal wiring layout.
The above conventional method is described in detail as follows.
Referring to FIG. 1A, on a substrate 100 comprising a MOS device, a dielectric layer 102, for example, a one-layered or two-layered spin-on-glass (SOG) sandwich type dielectric layer, is formed. The sandwich type dielectric layer prevents the formation of voids during by chemical vapor deposition (CVD).
In FIG. 1B, a photo-resist layer is formed and patterned on the dielectric layer. The dielectric layer 102 is etched to form a via hole 104, so that a gate cap, for example, titanium silicide or aluminum titanium, of the MOS is exposed within the via hole 104.
Referring to FIG. 1C, to improve the adhesion between the subsequently formed tungsten plug and the gate cap, a titanium nitride or titanium tungsten layer is formed. In addition to improve the adhesion, the titanium nitride or titanium tungsten layer is also used as an etch stop due to the difference of plasma spectrum between tungsten and titanium nitride or titanium tungsten. In practical application, titanium nitride is accompanied with titanium as a form of Ti/TiN. Therefore, before the formation of the tungsten plug, a titanium layer 106 is formed as a glue layer.
In FIG. 1D, a titanium nitride layer 108 having a thickness of about 800 .ANG. to 1200 .ANG. is formed, for example, by PVD. Normally, by PVD, a titanium layer is formed by DC magnetron sputtering. The titanium layer is then displaced in an environment containing nitrogen or ammonia. By rapid thermal process (RTP), the titanium is transformed into titanium nitride. Or a reactive sputtering is used to deposit titanium nitride. A mixture of argon and nitrogen is used as a reactive gas. Through ion bombardment, the titanium sputtered from a titanium target by ion sputtering is reacted with the reactive gas to form a titanium nitride layer.
In FIG. 1E, a refractory and well conductive metal layer 110, for example, a tungsten layer, is formed on the titanium nitride layer 108.
In FIG. 1F, apart of the metal layer 110 is removed to form a metal plug within the via hole 104. If tungsten is used for forming the plug, a gas containing carbon fluoride and oxygen, nitrogen fluoride and oxygen, or sulfur fluoride is used as the etchant. The tungsten is etched back with the titanium nitride layer 106 as an etch stop to form a tungsten plug.
In the conventional process mentioned above, if the titanium nitride barrier layer is formed by DC magnetron sputtering, titanium is reacted with the reacting gas in a high temperature. Since the step coverage provided by sputtering is limited, most of the titanium nitride is deposited on the surface of the via hole. Therefore, an overhand is formed as shown in FIG. 1G. As the integration of device increases, the problem becomes more and more serious. A sputtering technique of collimator is developed to improve the step coverage. The sputtering technique of collimator is used in a design rule of or under 0.25 .mu.m. However, though wide angle sputtering is avoided to improve the step coverage by using the collimator deposition, the titanium nitride deposition on the side wall of the via hole becomes very thin as shown in FIG. 1H. The adhesion between the subsequently formed tungsten plug and the titanium nitride barrier is degraded. Consequently, while forming tungsten plug, the reacting gas, tungsten hexafluoride, is reacted with the side wall of the via hole to form a titanium tetrafluoride (TiF.sub.4). The electrical characteristics are thus altered.